1. Field of the Invention
This invention relates to a control system for an automatic transmission, especially to a control system for an automatic transmission having a primary shifting unit and a secondary shifting unit connected to the primary shifting unit and capable of achieving multi-stage speed ranges.
2. Description of the Related Art
In an automatic transmission with an overdrive mechanism, 4-forward/1-reverse speed stages have heretofore been achieved by combining a secondary shifting unit, which includes an overdrive planetary gear unit, with a 3-forward/1-reverse primary shifting unit.
In the above case, the secondary shifting unit is constructed of an input shaft for inputting rotation obtained at a torque converter, a planetary gear unit, an overdrive brake for connecting or disconnecting a sun gear of the planetary gear unit relative to the housing of the automatic transmission, an overdrive direct clutch for connection or disconnection between the sun gear and a carrier, an output shaft connected to a ring gear, etc. The planetary gear unit can be locked up by bringing the overdrive brake into engagement with the housing of the automatic transmission to effect acceleration and hence to connect the overdrive direct clutch.
The primary shifting unit, on the other hand, is provided with two planetary gear units. Individual elements of both the planetary gear units are selectively connected through engagement or disengagement of corresponding brakes and clutches, whereby 3-forward/1-reverse speed stages are obtained.
In the speed stages ranging from the first speed to the third speed, the overdrive planetary gear is kept locked up; in the. fourth speed stage the overdrive planetary gear unit is brought into an accelerated state, whereby 4-forward/1-reverse speed stages are achieved.
The brakes and clutches, which are adapted to selectively connect the individual elements of both of the above-described planetary gear units, are connected or disconnected by feeding or draining hydraulic fluid to or from their corresponding hydraulic servomotors. A hydraulic circuit is therefore arranged so that pressure-regulated fluids are fed or drained to or from the respective hydraulic servomotors by changing over various valves.
To reduce shift shocks and also to diversify shift characteristics in the above automatic transmission in view ever-increasing engine torque, automatic transmissions have been proposed in which an accelerating function of an overdrive planetary gear unit is added to achieve 5-forward/1-reverse speed stages (see, for example, U.S. Pat. No. 4,733,580 issued Mar. 29, 1988 to Kubo et al.). In this case, for example, a new second speed stage is incorporated between the conventional first and second speed stages so that the conventional second, third and fourth speed stages are changed to the third, fourth fifth speed stages, respectively. In the new second speed stage, the primary shifting unit is maintained in the state of the first speed stage while in the secondary shifting unit, the overdrive brake and the housing of the automatic transmission are engaged with each other to achieve acceleration.
However, the above-described conventional automatic transmission may perform a combined shift wherein one of the primary shifting unit and the secondary shifting unit performs a downshift and the other executes an upshift. If a decision to execute a second shift is made in the course of the execution of the first shift, a shift shock may occur.
Here, a shift such that either one of the primary shifting unit and the secondary shifting unit performs a downshift and the other executes an upshift is defined as a "combined shift".
When the speed of revolution of an engine reaches a shift point for another speed stage on the high-speed side, for example, as a result of a reduction in engine load during the combined shift, a decision is made as to whether a second shift is needed or not. Where the other speed stage is a speed stage requiring reconfiguration of the secondary shifting unit into its accelerating state again, a first frictional engagement element which is being released in the above-described combined shift is brought into engagement again. The engaging hydraulic pressure for the first frictional engagement element temporarily drops, leading to occurrence of a shift shock.
In other words, in the combined shift described above, the first frictional engagement element is released to bring the secondary shifting unit into a lockup state and a frictional engagement element is engaged. The primary shifting unit is hence brought into an accelerating state. If the release of the first frictional engagement element in the secondary shifting unit takes place after the engagement of the second frictional engagement element in the primary shifting unit in the above instance, the downshift of the secondary shifting unit is delayed. Overall, as a total mechanism, a downshift is performed once an up-shift is effected, thus creating a shift shock.
The transmission is, therefore, designed that upon releasing the first frictional engagement element, the hydraulic servomotor for the first frictional engagement element is disconnected from its corresponding accumulator and the second frictional engagement element is engaged while controlling the hydraulic pressure in the hydraulic servomotor.
If the decision for the second shift is made while the combined shift is in progress by such a control, it becomes necessary to engage the first frictional engagement element again while the fluid in the hydraulic servomotor is drained to release the first frictional engagement element. As a result, the once-disconnected accumulator is connected again. At this time, the fluid in a line is fed to the accumulator which has begun to operate. The hydraulic pressure in the hydraulic servomotor therefore drops suddenly although this drop is temporary. Overall, the effect on the whole automatic transmission of the rapid downshift during the combined shift hence makes the shift shock greater.